Overview of Web Services

Extensible Markup Language (XML), developed by the World Wide Web Consortium (W3C) is one of the foundations of web services technology. XML enables web services and clients to communicate with each other in a common language. XML is a simple, flexible, text-based markup language in which data are marked using tags enclosed in angle brackets. Such markup allows different systems to easily exchange data with each other.

A web service is an application accessed by clients using XML-based protocols, such as Simple Object Access Protocol (SOAP), sent over internet protocols such as Hypertext Transfer Protocol (HTTP). Clients access a web service application through its interfaces and bindings, defined using XML artifacts such as a web services Definition Language (WSDL) file.

A Document Type Definition (DTD) or XML Schema Definition (XSD) describes the structure of an XML document. It has the tags an XML document can have, the order of those tags, and so on.

Extensible Stylesheet Language Transformation (XSLT) transforms XML documents from one format to another.

Web Services Standards

Simple Object Access Protocol (SOAP) provides a common messaging format for web services. SOAP enables objects not known to one another to exchange messages. SOAP uses an XML-based data encoding format and HTTP to transport messages. SOAP is independent of both the programming language and the operational platform, and it does not require any specific technology at its endpoints.

ebXML (Electronic Business using eXtensible Markup Language) is a set of specifications that enables enterprises to conduct business over the Internet. OASIS (Organization for the Advancement of Structured Information Standards) controls the ebXML specifications. Similar to a telephone system's yellow pages, an ebXML registry enables providers to register their services and requestors to find services. Once a requestor finds a service, the registry has no more role to play between the requestor and the provider.

Universal Description, Discovery, and Integration (UDDI) provides a standard way to register, de-register, and look up web services.

Web Services Description Language (WSDL) defines a standard way to specify the details of a web service. It is a general-purpose XML schema that can specify details of web service interfaces, bindings, and other deployment details. By having such a standard way to specify details of a service, clients who have no prior knowledge of a web service can use it.

Java EE Web Service Standards

Java API for XML-based remote procedure calls (JAX-RPC) uses an XML-based protocol for client-server remote procedure calls. JAX-RPC enables SOAP-based interoperable and portable web services. Developers use the JAX-RPC programming model to develop SOAP-based web service endpoints, along with corresponding WSDL descriptions, and clients. A JAX-RPC based web service can interact with clients that are not based on Java. Conversely, a JAX-RPC based client can interact with a non-Java-based web service implementation.

Java API for XML Web Services (JAX-WS) 2.0 is the successor to JAX-RPC that provides an “integrated stack” of APIs for developing web services, including:

• JAXB 2.0-based data binding.

• Support for the latest W3C and WS-I standards such as SOAP 1.2 and WSDL 1.2.

• Ease-of-development features using annotations.

• An improved handler framework, support for asynchronous RPC and non-HTTP transports.

• JSR 109 support.

SOAP with Attachments API for Java (SAAJ) enables developers to produce and consume messages conforming to the SOAP 1.1 specification and SOAP with Attachments note. SAAJ provides an abstraction for handling SOAP messages with attachments. Advanced developers can use SAAJ to have their applications operate directly with SOAP messages. Attachments may be complete XML documents, XML fragments, or MIME-type attachments. In addition, SAAJ allows developers to enable support for other MIME types. JAX technologies, such as JAX-RPC, internally use SAAJ to hide SOAP complexities from developers. SAAJ enables:

• Synchronous request-response messaging: the client sends a message and then waits for the response.

• One-way asynchronous messaging: the client sends a message and continues with its processing without waiting for a response.

JSR 109 (Implementing Enterprise Web Services) facilitates building interoperable web services in the Java EE environment. It standardizes the deployment of Web services in a Java EE container. JSR-109 builds upon JAX-RPC to define a standard mechanism for deploying a web service in Enterprise JavaBean (EJB) and servlet containers. Application Server supports managing JSR 109 applications based on either JAX-RPC or JAX WS 2.0.

JSR 181 (Web Services Metadata for the Java Platform) defines an annotated Java format that uses Java Language Metadata to enable easy definition of Java web services in a Java EE container.